Harmonic Structure (HS) has a heterogeneous microstructure consisting of bimodal grain size together with a controlled and specific topological distribution of fine and coarse grains. In other words, the HS is heterogeneous on micro- but homogeneous on macro-scales. The most unique feature of HS is its continuously connected UFG Shell structure. In the present work, the HS design has been applied to pure metals and alloys via SPD powder metallurgy process. At a macro-scale, the harmonic structure materials exhibited significantly better combination of strength and ductility, as compared to their homogeneous microstructure counterparts. This behavior is essentially related to the ability of the HS to promote a large strain hardening and uniform distribution of strain during plastic deformation, leading to improved mechanical properties by avoiding or delaying localized plastic instability. Such outstanding mechanical properties of HS materials are attributed to the continuously connected UFG-Shell structure. This unique microstructure provides stress partitioning in the micro scale and stress dispersion in the macro scale.

Our strategy to enhance fracture properties of ultra-high-strength low-alloy steel with a yield strength of 1.4 GPa or over is to arrest the propagation of brittle crack in a hierarchical, anisotropic and ultrafine grain structure designed to be fail-safe, in addition to suppressing the crack initiation. The present article reviews strength, ductility, toughness and delayed fracture resistance of ultra-high-strength low-alloy steel with an ultrafine elongated grain structure that was processed by deformation of a tempered martensitic structure at an elevated temperature (warm tempforming). The evolution of heterogenous microstructure during warm tempforming using multi-pass caliber rolling and the microstructural factors controlling the strength and the fracture properties of the warm tempformed steels are discussed. Furthermore, we introduce the application of the warm tempformed steel with an ultrafine elongated grain structure to ultra-high-strength bolt.

Strain hardening behavior of ferrite layers in the microstructure of drawn pearlite wire is studied theoretically and numerically. It is shown that stress field associated to dislocations could diminish quickly if the dislocations enter the phase or grain boundaries and decompose into smaller segments to distribute along the boundary. Some atomistic simulations of single-phase media validate this phenomenon; dislocations show to pass, decompose or accumulate on tilt-type grain boundaries depending on their atomistic configuration. Mechanical responses of nine-layered pearlite models subjected to tensile load are analyzed by a strain gradient crystal plasticity finite element code, where possible passage or absorption of dislocations is expressed in the model of dislocation mean free path. The critical resolved shear stress for slip systems consists of the lattice friction, the Taylor and Orowan terms and the strain hardening is given by the Taylor one. The density evolution of accumulated dislocations is evaluated by the model of Kocks and Mecking where the dislocation mean free path plays a major role. Results show that the smaller the dislocation absorption ability of the phase boundary and thinner the layer thickness, larger the strain hardening becomes. Slip localization in cementite layers is shown to be suppressed when the strain hardening of ferrite layers is higher, and this trend is consistent with results obtained in previous studies by molecular dynamics simulation and classical elasto-plasticity analyses. Scale sensitive phenomena taking place at phase boundaries in layered structure are briefly discussed in views of atomistic process and continuum mechanics.

Fully pearlitic steel was wire-drawn up to the strain of 2.2. Torsion tests were performed using two types of specimen; one was as-drawn specimen, and the other was aged at 423 K for 3.6 ks. A delamination crack propagated along the longitudinal direction of the wire in the aged specimen while it showed normal fracture perpendicular to the longitudinal direction in the as-drawn specimen during the torsion tests. Backscattered electron images indicated that the cementite lamellae beneath the delamination crack had vanished while cementite lamellae beneath the normal fracture surface had rotated until the fracture in the as-drawn specimen. Torsion tests with different stain rates indicated the inverse strain-rate dependence of the onset of the delamination. Those suggest that the plastic deformability of ferrite and the existence of the thermally activated process which controls the cementite dissolution are the points for the onset of the delamination. In the present study, the effect of aging and deformability of ferrite on delamination is discussed, suggesting that the delamination crack propagates as the result of the local plastic instability in the scale of several microns.

The relationship between yield stress and the distribution of microscopic plastic deformation was numerically investigated by using a crystal plasticity finite element method (CP-FEM) in the model where particles were randomly distributed. It was in order to reveal which particle spacing. i.e., the maximum, minimum or average particle spacing, can be taken as the representative length which controls yielding. The critical resolved shear stress for the onset of the slip deformation in any element was defined under the extended equation in the Bailey-Hirsch type model. The model includes the term of the Orowan stress obtained from the local values of the representative length. Each particle spacing was distributed with a standard deviation of approximately 2 to 3 times larger than the average particle spacing. The macroscopic mechanical properties obtained with CP-FEM were in good agreement with those experimentally obtained. The onset of microscopic slip deformation depended on the particle distribution. Plastic deformations started first in the area where the particle size is larger, then the plastic region grows in the areas where the particle spacing is smaller. Slip deformation had occurred in 90% of the matrix phase by the macroscopic yield point. The length factor in the Orowan equation was the average spacing of the particles in the model, which is in good agreement with Foreman and Makin. The CP-FEM indicated that in dispersed hardened alloys, microscopic load transfer occurred between the areas where the large particles spacing and the small one at the yielding.

Toward a high demand for ultrahigh strength automotive components for reducing body weight and increasing crashworthiness, hot stamped components have been employed worldwidely, and the further increase in strength of hot stamped components is being requested. A large obstacle of the strength increase is the concern about the occurrence of delayed fracture.

In this study, we investigated the effects of microstructure on the resistance to delayed fracture of hot stamped steel sheets from a viewpoint of the control of heterogeneous microstructure, and the following results were obtained:

(1)The grain refinement of martensite increases the resistance to delayed fracture.

(2)Samples containing some amount of ferrite in the martensite matrix have a lower or higher resistance to delay fracture than samples with full martensitic microstructure, depending on the morphology of ferrite.

(3)The presence of residual austenite of several % increases the resistance to delayed fracture remarkably but if it is strained and transformed to martensite, the resistance to delayed fracture drastically decreases.

(4)Concerning 2000 MPa class hot stamping steel sheets, a higher carbon steel quenched and tempered at low temperature has higher resistance to delayed fracture than a quenched lower carbon steel with the same strength.

In the paper, we discussed these results including their mechanisms in detail.

Particle dispersion ferritic steels with hard VC carbide particles and that with soft Cu particles were tensile-tested for investigating the effect of particle nature (hard or soft) on plastic deformation and ductile fracture behaviors. The Cu dispersion steel exhibited a significantly larger necking deformation than the VC dispersion steel, which is due to less frequent void formation in the Cu dispersion steel. Digital image correlation (DIC) analysis for tensile-deformed specimens revealed that the plastic strain was concentrated around the VC particles in the VC steel, while that was distributed within ferrite matrix away from Cu particles in the Cu steel. As a result of high-magnification observation for the void formation in each steel, it was found that nano-sized voids were nucleated at the interface of the rigid VC particles, while they were never formed at the plastically-elongated Cu particles. Reduced stress/strain concentration at the particle interface is inferred to be occurred by the plastic deformation of soft Cu particles during the tensile deformation. This leads to the retardation of ductile fracture to the higher stress/strain regime and the superior local ductility in the Cu steel.

Erichsen and deep drawing tests were performed to examine the formability of 0.2% carbon steel sheets containing the bimodal microstructure fabricated by 70-75% heavy-reduction controlled rolling process with a range of heating temperatures from 700-1000°C. The formability of steel sheets was analyzed and discussed in terms of the microstructure, the crystallographic texture, the strain-hardening exponent, and the anisotropy coefficient, which are obtained from the tensile test. In Erichsen test, Erichsen Index (EI) of 900- and 1000°C-heated specimens with a bimodal structure of submicron-sized grains (< 1 μm) and micron-sized grains (2-5 μm) is similar to that of as-received specimen with an average grain size of 41 μm, meaning superior formability. In deep drawing test, failure took place in all 700°C-heated specimens, while others could be drawn with a drawing ratio of 2.0. 900- and 1000°C-heated specimens were uniformly deformed without ears.

Ultrafine ferrite + austenite steels with the chemical composition of 0.1%C-2%Si-5wt%Mn show excellent strength (TS=1200 MPa) and high ductility (TEl=25%) balance, compared to conventional TRIP steels. This steel is expected as the third generation AHSS. This steel can be produced by a simple intercritical annealing, however, longer annealing time is necessary to obtain appropriate ferrite + austenite structure. It is difficult to produce this steel by continuous annealing process. If the annealing time can be drastically reduced, this new TRIP steels can be commercialized. We focused on the effect of the prior microstructures before annealing on the formation of ferrite + austenite structure. The effect of the prior structure is not clear. Therefore, in this study, two kind of prior structures, ultrafine grained ferrite + cementite and martensite were used in 0.1%C-2%Si-5wt%Mn steels. It was found that the prior structure of ferrite + cementite can form large amount (20%) of austenite in a very short time (600 s). This is because cementite finely dispersed in the structure effectively acts as a preferential nucleation site of reverse transformed austenite and C and Mn are concentrated in cementite to enable a short time formation of austenite. Excellent strength-ductility balance (32000 MPa%) which is superior to conventional TRIP steels is also obtained.

This paper presents an introduction to the electrochemical properties at the boundary between MnS and steel matrix of Type 304 stainless steel. It starts with the information about the corrosion mechanism from trenching to pitting at the boundary. The proposed mechanism is as follows: 1) MnS dissolution leads to the deposition of elemental sulfur on and around the inclusions; 2) the coexistence of S and Cl^– ions results in the dissolution of the steel matrix side of the boundary, forming the trenches along the periphery of the inclusions, 3) rapid active dissolution occurs locally at the bottom of the trenches, because of the decrease of pH due to the hydrolysis reaction of Cr³⁺ and the electrode potential decrease at the bottom of the trench due to ohmic drop. The prevention of MnS dissolution is effective to inhibit the pit initiation. The oxide film was found to be generated on the MnS inclusions by the exposure to humidified air for 30 and 90 days, and it was confirmed that the oxide films effectively inhibit the inclusion dissolution and the trench formation at the boundaries, making the inclusions less active as the initiation sites for pitting. The suppression of the active dissolution of the steel matrix is also effective to prevent the pitting. No pit initiation occurred at the MnS inclusions in low-temperature carburized stainless steels. It was clarified that the interstitial carbon sufficiently inhibits the active dissolution rate of the steel matrix.

Fatigue crack growth characteristics of an Fe20Cr20Ni20Mn20Co high entropy alloy (HEA) were investigated by ΔK increasing compact tension test in comparison with a SUS316L. Fatigue crack growth rate of the HEA was lower than that of the SUS316L. The predominant crack growth path was grain interior for both alloys. A difference was found to be in the crack roughness, namely, the fatigue crack growth path of the HEA was more distinctly deflected compared with that of the SUS316L. This fact indicates that roughness-induced crack closure is a key factor decreasing the crack growth rate of the HEA. Another key is a non-crystallographic transgranular crack growth mechanism. The SUS316L shows crack growth via crack blunting/re-sharpening, while the HEA shows transgranular crack growth associated with dislocation substructure alignment.

We measured local grain orientation gradient and dislocation density by electron backscatter diffraction (EBSD) measurement and electron channeling contrast imaging (ECCI) to obtain strain maps near a stress concentration source in a pure nickel as a FCC model specimen. In particular, we obtained relationship among grain orientation spread (GOS), dislocation density, and equivalent plastic strain on the specimen surface, which were obtained by EBSD, ECCI, and digital image correlation (DIC), respectively. After obtaining GOS-strain and dislocation density-strain relations, the strain distribution in the specimen interior was also determined by measuring the GOS and dislocation density. Both of the GOS and dislocation density showed a linear correlation, and the dislocation density-strain relation showed a relatively small deviation.

The effects of Mn, Cr, and Ni addition on the hydrogen diffusion behavior in BCC, FCC, and HCP Fe was investigated by means of first-principles calculations. Diffusion coefficients were estimated quantitatively from the migration energy calculated by the nudged elastic band method and the vibrational energy at every stable and metastable site. The addition of Mn, Cr, and Ni to a BCC lattice has a blocking effect on hydrogen diffusion and decreases the diffusion coefficient of hydrogen. Crystal orbital Hamilton population (COHP) analysis revealed that the weakened bonding between the added element and hydrogen is the origin of the blocking effect. On the other hand, the addition of Mn, Cr, and Ni to FCC and HCP lattices resulted in the formation of hydrogen trap sites. In the FCC case, the diffusion coefficients of Fe₃₁MnH, Fe₃₁CrH, and Fe₃₂H, showed similar values, while that of Fe₃₁NiH was lower. In the HCP case, the diffusion coefficients of the three additional elements showed a decreasing trend. Based on the results of the COHP analysis, we conclude that the octahedral interstitial sites around the additional elements become trap sites in FCC and HCP Fe due to the strengthened bonding between Fe and H.

This review reports the research activity on the hydrogen embrittlement in high-strength aluminum alloys, especially focusing on hydrogen trapping at various trap sites and its influence on hydrogen embrittlement. We have investigated the three representative hydrogen embrittlement mechanisms in high-zinc-concentration Al-Zn-Mg alloys. One of the three mechanisms is the damage evolution originated from hydrogen precipitated as pores. We have paid marked attention to the existence of age-hardening precipitates as the major hydrogen trap site.

Firstly, we have clarified the nanoscopic structures of a few MgZn₂ precipitates and their interface by means of the high-resolution TEM technique. Such information has been utilized to perform a first principles simulation to know trap binding energy values for almost all the possible trap sites. At the same time, detailed fracture micromechanisms and microstructure-property relationships have been investigated by employing both the high resolution X-ray micro-tomography technique and the first principles simulation. The ultra-high-resolution X-ray microscope, which has been realized quite recently, has also been applied. Characteristic localized deformation and subsequent crack initiation and growth through deformed aluminum have been observed. It has also been revealed that hydrogen embrittlement has been suppressed when relatively coarse particles are dispersed. In-situ hydrogen repartitioning during deformation and fracture has been estimated by considering thermal equilibrium among the various trap sites together with the increase in trap site density during deformation. The relationship between the in-situ repartitioning of hydrogen and hydrogen embrittlement with the three different micromechanisms are discussed to explain realistic conditions for hydrogen embrittlement to occur.

Evolution of heterogeneous nano-structure in heavily cold-rolled SUS316LN stainless steels was investigated in detail. Transmission electron microscopic observations from the transverse direction (TD) of the 92% rolled specimen revealed the formation of a typical hetero-nano structure composed of ultra-fine lamellae embedded with deformation twin domains. The twin domains had prolate ellipsoidal shape elongated parallel to TD. Two types of twin domains with different crystallographical orientations to matrices could be identified, i.e., i) <211> // rolling direction (RD) and <110> // TD or ii) <110> // RD and <211> // TD, although all the {111} twining planes of both twin domains were oriented nearly parallel to the rolling planes. The ultra-fine lamellae were elongated along <100> direction and nearly parallel to RD. Deformation twins with a few nano-meter spacing were also frequently observed to develop in the lamellae. Evolution sequence of the hetero-nano structure during cold rolling was also investigated. At an early stage of rolling, deformation twins were gradually formed in the whole grains. Then, the regions fragmented grains by twins were further subdivided by a numerous number of shear bands inclined at about 20~45° from the RD, resulting in the formation of “eye-shaped” twin domains surrounded by shear bands and their crystallographical rotation. Cold rolling up to 50% caused a considerable increase in strength and decrease in ductility. While the strength was raised more with increasing reduction up to 92%, both the strength and ductility eventually slightly decreased by further rolling.

Severe plastic deformation has attracted interests as one of the breakthrough procedures to improve various properties of metals and alloys. Recently, it has been revealed that heavy cold rolling of some kinds of austenitic stainless steels can cause ultrafine-grained structure comparable with those achieved by severe plastic deformation. Coarse initial grains were fragmented by deformation induced microstructure to develop heterogeneous nanostructure. Tensile strength of heterogeneous-nanostructured stainless steel exceeds 2 GPa. It is considered that high strength of heterogeneous-nanostructured metals is attributed to such peculiar microstructure with dispersed “eye-shaped twin domains”. In this study, microstructural mechanisms and factors which contribute to macroscopic strength of heterogeneous-nanostructured austenitic stainless steel were evaluated on the basis of multiscale crystal plasticity simulation. Microstructure of heavily cold-rolled SUS316LN austenitic stainless steel was investigated by transmission electron microscopy, and stress-strain curves were attained by tensile tests. It was observed that microstructure of SUS316LN manufactured by 92% cold rolling was composed of deformation nano-twins, shear bands, and lamella structure. Evaluation of mechanical properties of heterogeneous-nanostructured SUS316LN was conducted using crystal plasticity finite element simulation considering microstructural information, such as dislocation density, crystal orientation, shape of grains, and dislocation sources. Information of microstructure obtained by electron backscatter diffraction, e.g. geometry of heterogeneous nanostructures and crystal orientation, were introduced to computational models for multiscale crystal plasticity simulation. It was revealed that deformation behavior depends on the tensile direction and the strength increases with the increase of volume fraction of twin domains as well as nano-twin and lamellar inter-spacings.

Fatigue behaviors of duplex stainless steels with excellent tensile properties brought by heterogeneous nanostructure were examined with changing volume fraction of ferritic-austenitic phases. In as-rolled samples, fatigue strength taken by 10⁷ cycles became double comparing with those of conventional stainless steels. The volume fraction of ferritic-austenitic phases has not affected fatigue behavior. Further, the fatigue strength was improved additionally 200 MPa by aging heat-treatment after cold rolling. The duplex stainless steels with heterogeneous nanostructure possessed enough fatigue strength even in 3%NaCl solution though its fatigue strength declined.

The effect of lattice defects on the tribological behavior for high friction coefficient under tricresyl phosphate (TCP) added poly-α-olefin (PAO) lubrication was investigated in the nanostructured steels produced by heavy plastic deformation processes. In the surface-nanostructured SUJ2 bearing steel, the tribological behavior with high friction coefficient was observed in the ball-on-disk tests in comparison with the non-deformed steel. In addition, the similar phenomenon was observed in the ultra-low carbon (ULC) steel with high-density of lattice defects (grain boundary, dislocation and so on). By increasing the density of lattice defects, higher friction coefficient was shown. The reason of the tribological behavior with high friction coefficient seems that the compound film of Fe-O-P system formed in the ball-on-disk test was worn.

Time-resolved and in-situ observations using synchrotron radiation X-rays successfully proved that the massive-like transformation, in which the γ phase was produced through the solid – solid transformation and the partition of substitute elements such as Mn and Si at the δ/γ interface could be negligibly small, was selected in the unidirectional solidification of 0.3 mass%C steel at a pulling rate of 50 μm/s. The massive-like transformation produced fine γ grains in the vicinity of the front of δ/γ interface. The coarse γ grains also grew behind the fine γ grains along the temperature gradient. Distance between the δ/γ front and the advancing front of coarse γ grains was as short as 200 μm. Namely, the fine γ grains disappeared within 10 s by the growth of coarsen γ grains along the temperature gradient. In addition, the observation of the δ/γ interface confirmed that a transition from the diffusion-controlled γ growth to the massive-like growth of γ phase occurred at a growth rate of 5 μm/s. Thus, the massive-like transformation is dominantly selected in the carbon steel during conventional solidification processes.

Microstructures and crystallographic features of layered double hydroxide (LDH) particles and the LDH film formed on the aluminum alloy sheet (alloy 7075) using the steam coating process were examined by transmission electron microscopy (TEM). The LDH powder particles exhibited a hexagonal-disc-shape morphology. Their disc plane and side planes were parallel to a basal plane and prism planes of the LDH hexagonal crystal, respectively. A cross-sectional thin TEM sample was prepared from the interface between the LDH film and the Al alloy substrate using a focused ion beam combined with pick-up technique. TEM observations revealed that the LDH film formed on the Al alloy exhibited a dual-layer structure, which consists of a continuous Zn-rich amorphous layer (including numerous nano-particles with fcc structure) on the Al substrate side and a LDH layer composed of a number of disc-shaped LDH particles.

Effect of interphase boundary diffusion of carbon on pearlite transformation in Fe-C system is investigated by phase-field simulations. The model considers volume diffusion in α, γ and θ phases and boundary diffusion at α/γ and γ/θ interfaces. Growth velocity of pearlite at eutectoid composition is simulated under various conditions of interlamellar spacing at 873, 898 and 923 K. The simulation results show that growth velocity has a maximum value (ν) at an interlamellar spacing (λ) at each temperature. The simulated values of ν and λ at different temperatures satisfy the equation νλⁿ=const.; the value of exponent n increases with increasing the value of boundary diffusion coefficient. This result suggests that the value of boundary diffusion coefficient can be estimated from phase-field simulations and experimental value of n. The ratio of volume diffusion flux to boundary diffusion flux (R_v/b) during the formation of pearlite is approximated from the values of volume diffusion coefficient in the γ phase, boundary diffusion coefficient and maximum growth velocity at each temperature. The value of R_v/b is small at 873 and 898 K while it is large at 932 K. It seems that the rate-controlling process of pearlite transformation changes from volume diffusion to interface diffusion with decreasing temperature at 873-932 K.

For deeper understanding of a dynamic accommodation mechanism of internal stress in pearlite originated from the lattice misfit between ferrite and cementite phases, the lattice parameter ratios of cementite, b_θ/a_θ and c_θ/a_θ, were locally analyzed in detail by using the electron backscatter diffraction (EBSD) technique. The EBSD analysis has revealed that lattice parameter ratios of cementite lamellae obviously differ from those of spheroidized cementite particles, which demonstrates that pearlite has a certain amount of internal stress as long as it maintains lamellar structure. The internal stress in pearlite gradually decreased during isothermal holding at 923 K after pearlitic transformation due to interfacial atomic diffusion of iron atoms. However, comparing with theoretical values under Pitsch-Petch orientation relationship, it was understood that large amount of internal stress had been already accommodated upon pearlitic transformation by introduction of misfit dislocations and structural ledges on ferrite/cementite lamellar interfaces. That is, the internal stress of pearlite is dynamically reduced by two different processes; built-in accommodation upon pearlitic transformation and additional time-dependent relaxation after pearlitic transformation. On the other hand, EBSD analysis and neutron diffraction technique gave remarkably different lattice parameters of cementite. From this result, it is concluded that various crystallographic orientation relationships between ferrite and cementite coexist in pearlite. Furthermore, elastic strain energy analysis suggests that the invariant-line criterion on ferrite/cementite interface plays an important role for the selection of orientation relationships in pearlite.

The alloying effects of Ti, which is strongly attracted to nitrogen (N), on nitride precipitation and the resultant surface hardening in nitrided Fe-Al-Ti alloys, were investigated using three-dimensional atom probe tomography. Ti addition promotes nucleation of Al nitrides, which leads to substantial surface hardening. As the amount of added Ti increases, the hardness near the surface increases because the nucleation of Al nitrides is promoted, likely due to the higher density of Ti-N clusters. However, the effect of Ti on surface hardening by nitriding in Fe-Al alloys is weaker than that of V addition, as reported previously, although Ti is a stronger cluster-forming element than V. These results suggest that, in addition to the ability to form clusters, the potency of clusters for AlN nucleation needs to be considered to control AlN precipitation by solute clustering.

The effective cluster interactions (ECIs) in the Fe-Ti-N system were evaluated using the cluster expansion method. The ECIs were estimated under two different conditions. One condition was to evaluate the ECIs based on configurational energies of ordered structures in which each volume was fully relaxed. Another one was to obtain the ECIs from the energies of the structures with each volume was fixed to that of pure Fe. Then, these interactions were utilized in the free energy calculation and Monte Carlo (MC) simulation. Two-phase separation tendency between Fe and TiN was observed in the free energy calculation and the clustering behavior of Ti and N in the Fe bcc matrix was confirmed by the MC simulation. Thus, the result strongly suggests that the i-s clustering is caused by two-phase separation.

According to the MC simulation based on the interaction energies obtained by the condition of fixed volume to pure Fe, layered shaped i-s clusters appears. On the other hand, the MC simulation based on the interaction energies in which volumes were relaxed shows sphere shaped i-s clusters. Thus, elastic constraint from Fe lattice strongly influences the shape of i-s nanoclusters in the very first stage of annealing. Furthermore, energies were compared in structure models with different coordination of N atoms around Ti atoms. The result shows that the clusters of Ti:N = 1:1 preferably form when the tetragonal distortion of the matrix is small. However, when the tetragonal distortion is large, the clusters of Ti:N = 1:3 become stable.